AQ 232 – Fish Population Dynamics and Stock Assessment

The Unit Stock

Nyamisi Peter

2026-01-21

Introduction

  • Fish stock assessment was developed in the temperate regions where there are very few species of fish.

  • It was based on a single species

  • However, such a consideration is not very applicable for tropical fisheries where we have multi-species fisheries because;

    • Different species are mixed up
    • There is very high interaction among species
  • In such situation, fish populations are divided into stocks (unit stock) for management and assessment purposes

  • Understanding the distribution, abundance, and methods for estimating stock parameters is important for effective stock assessment and sustainable fisheries management.

Unit Stock

  • A unit stock is a population of fish, either of a single or different species, grouped together for assessment purposes which may or may not include all the fish in a stock.

  • A unit stock is a group of fish that:

    • Share a common gene pool – can interbreed
    • Occupy a defined geographical area – have a specific distribution
    • Exhibit similar life history traits – growth, reproduction, mortality
    • Are subject to similar fishing pressures – exploited by the same fisheries
    • Are managed as a single entity – for conservation and exploitation, – Can be managed independently from other stocks

Determination of a Unit Stock

  • Unit stocks can be determined through various biological and ecological characteristics, including:

    • Distribution patterns – Where unit stocks are located
    • Spawning behavior – Different stocks spawn in different areas
    • Population parameters – Recruitment, growth and mortality rates, morphometric differences
    • Physiological traits – Temperature at hatching, vertebrae structure, scale decorations, parasite types

Distribution and Abundance

Geographic Distribution

  • Geographic distribution refers to the spatial area where a fish stock is found, including the boundaries of its range.

Key concepts:

  • Fish populations are not uniformly distributed across all suitable habitats
  • Distribution patterns change seasonally (spawning, feeding, wintering areas)
  • Some species have discrete distributions while others have overlapping ranges
  • Distribution limits are determined by temperature, salinity, bathymetry, and food availability

Geographic Distribution

Examples:

  • Migratory species (salmon) move between coastal and riverine habitats
  • Demersal species (bottom-dwelling) have specific depth distributions
  • Pelagic species distribute widely in open water

Management implications:

  • Fishing effort should target areas of highest concentration
  • Seasonal closures may protect aggregations during vulnerable periods
  • International boundaries may complicate management of stocks crossing borders

Spatial and Temporal Variation in Abundance

Temporal Variation:

  • Seasonal changes - Abundance fluctuates with feeding, spawning, and migration cycles
  • Interannual variation - Year-to-year changes in recruitment and survival
  • Long-term trends - Decadal-scale changes related to fishing pressure or environmental conditions

Spatial and Temporal Variation…

Spatial Variation:

  • Patchiness - Fish concentrate in certain areas (spawning grounds, feeding areas)
  • Gradients - Density changes across depth, latitude, or habitat types
  • Hotspots - Areas of consistently high abundance

Spatial and Temporal Variation…

Measurement challenges:

  • Detecting aggregations requires spatially intensive sampling
  • Small-scale variability requires high sampling effort to quantify accurately
  • Temporal dynamics require time series of observations

Habitat Requirements and Factors Affecting Distribution

Abiotic Factors:

  • Temperature - Most critical factor; thermal preferences vary by species
  • Salinity - Freshwater, brackish, or marine preferences
  • Dissolved oxygen - Required for respiration; anaerobic zones are avoided
  • Bathymetry - Depth preferences related to light and pressure tolerance
  • Substrate type - Bottom type affects benthic food availability

Habitat Requirements…

Biotic Factors:

  • Food availability - Distribution follows food resources
  • Predation risk - Avoidance of predator-rich areas
  • Competition - Space partitioning with other species
  • Reproductive habitat - Essential for spawning and nursery grounds

Habitat Requirements…

Habitat Quality:

  • Optimal habitat supports higher population densities
  • Marginal habitat may be temporarily occupied but has lower reproductive success
  • Loss or degradation of critical habitat can limit population size

Environmental Influences on Stock Distribution

Physical Oceanography:

  • Ocean currents transport larvae and affect distribution
  • Upwelling events create productive feeding areas
  • Coastal current patterns structure population distributions

Environmental Influences…

Climate and Weather:

  • Climate change shifts suitable habitat ranges
  • Seasonal weather events (storms, cold snaps) cause temporary distribution changes
  • Long-term climate trends alter distribution boundaries

Straddling stock and highly migratory fish stock

  • With 1982’s United Nations Convention on the Law of the Sea (UNCLOS), coastal nations were given the right to manage fisheries within their Exclusive Economic Zones (EEZ)
  • But of course, fish don’t adhere to imaginary lines in the ocean.
  • The management of two groups – straddling and highly migratory fish stocks became problem after the 1982 Convention.

Straddling stock

  • Straddling stocks are fish stocks that migrate across more than one Exclusive Economic Exclusion Zone (EEZ) and the high seas
    • Straddling stocks is the stocks of fish which migrate between, or occur in both, the EEZ of one or more states and the high seas
  • Fish stocks that migrate through, or occur in more than one exclusive economic zone.
  • They include species such as swordfish and marlin.

Highly migratory fishes

  • Highly migratory fishes are the ones that travel long distances across international waters
  • These include tuna and tuna-like species such as yellowfin tuna, bigeye tuna, and skipjack.

Management of Straddling stock and Highly migratory fishes

  • Management of the straddling stock and highly migratory fishes is a shared responsibility between coastal states and the international community.

  • The 1995 United Nations Fish Stocks Agreement (UNFSA), an international treaty designed to further implement provisions of the 1982 Convention which;

    • Ensure the long-term conservation and sustainable use of straddling fish stocks and highly migratory fish stocks

Management…

  • Sovereign responsibility must be worked out in collaboration with neighboring coastal states and fishing entities.
  • Regional Fisheries Management Organizations (RFMOs) are established to manage these stocks collaboratively.
  • Transboundary stock range in the EEZs of at least two countries.
  • A stock can be both transboundary and straddling.

Examples from Tanzania Marine Waters

Tanzanian Exclusive Economic Zone (EEZ)

  • Tanzania has an EEZ of approximately 220,000 km²
  • Extends 200 nautical miles from the coastline
  • Bordered by Kenya (north), Mozambique (south)
  • Manages several important fish stocks
  • Home to both regional and international fisheries

Tanzania marine zones

1: Yellowfin Tuna (Highly Migratory)

Species: Yellowfin tuna (Thunnus albacares)

Distribution:

  • Found throughout Indian Ocean
  • Migrates across multiple EEZs (Tanzania, Kenya, Mozambique, Seychelles)
  • Extends beyond EEZs into high seas
  • Seasonal movements following monsoon patterns

Thunnus albacares

Yellowfin Tuna (Highly Migratory)

Management:

  • Managed by IOTC (Indian Ocean Tuna Commission)
  • Tanzania is member state of IOTC
  • IOTC sets total allowable catch (TAC) for entire Indian Ocean
  • Allocates quota to member countries
  • Tanzania receives annual catch quota

Thunnus albacares

2: Bigeye Tuna

Species: Bigeye tuna (Thunnus obesus)

Distribution:

  • Found in tropical/subtropical Indian Ocean waters
  • Deep-diving species (300-400 m daytime depth)
  • Migratory across multiple EEZs
  • Year-round presence in productive upwelling zones

Thunnus obesus

Bigeye Tuna

Fishery:

  • Caught by longlining (deep hooks target night feeders)
  • Tanzania vessels and foreign fleets participate
  • Increasing fishing pressure in recent decade

Thunnus obesus

Priority Fisheries in Tanzania

Tanzania’s fisheries are strategically important for food security, employment, and economic development. Priority ranking considers:

  • Economic value (export revenue)
  • Food security contribution (protein supply)
  • Employment (fisher livelihoods)
  • Management feasibility (data availability)

Priority Fisheries in Tanzania

  • Major priority fishery in Tanzania:

Marine

  1. Tuna and Tuna-like species (Yellowfin, Bigeye, Skipjack)
  2. Small Pelagics (Sardines, Anchovies)
  3. Octopus fisheries

Freshwater

Lake Victoria

  1. Nile Perch (Lates niloticus)
  2. Dagaa (Rastriger argentea)

Lake Tanganyika

  1. Cichlids (Lates stappersii),
  2. Sardines (Stolothrissa tanganicae, Limnothrissa miodon)

Mixed stock

  • A mixed stock occurs when individuals from two or more genetically distinct populations are found together in the same geographic area
  • Fish from multiple spawning populations intermingle during feeding, migration, or overwintering
  • Cannot be easily distinguished visually but are genetically or demographically separate

Common Scenarios for Mixed Stock

1. Feeding Aggregations

  • Fish from different spawning stocks congregate in productive feeding grounds
  • Example: Salmon from multiple rivers feeding together in the ocean

2. Migratory Overlap

  • Different populations share migration routes
  • Example: Tuna stocks from different ocean regions mixing along migration corridors

3. Seasonal Mixing

  • Populations separate for spawning but mix at other times
  • Example: Herring from different spawning grounds overwintering together

Mixed Stock: Management Challenges

Challenge Implication
Identification Difficult to assign individuals to source populations
Harvest allocation Risk of overfishing weaker stocks while targeting abundant ones
Stock assessment Complex models needed to account for mixing rates
Weak stock concern Less productive stocks may be depleted if managed as one unit

Mixed Stock: Identification Methods

Methods to identify and separate mixed stocks:

  • Genetic markers (microsatellites)
  • Otolith microchemistry (trace elements indicate natal origin)
  • Parasite tags (different parasites in different regions)
  • Morphometric analysis (body shape differences)
  • Tagging studies (track individual movements)

Mixed Stock: Example from East Africa

Lake Victoria Dagaa (Rastrelliger argentea)

  • The dagaa fishery may include mixed stocks from different lake regions
  • Fish aggregate in certain areas for feeding
  • Different populations have varying productivity levels
  • Requires careful management to avoid depleting localized populations

Indian Ocean Yellowfin Tuna

  • Mixing of stocks from Western and Eastern Indian Ocean
  • Genetic studies show population structure across the ocean basin
  • IOTC manages as a single stock but monitors regional differences

References and Further Reading

  • Quinn, T. J., & Deriso, R. B. (1999). Quantitative Fish Dynamics. Oxford University Press.
  • Hilborn, R., & Walters, C. J. (1992). Quantitative Fisheries Stock Assessment. Chapman & Hall.
  • Petitgas, P. (1998). Geostatistics in fisheries survey design and stock assessment. FAO Fisheries Technical Paper 368.

Last updated: January 2026